Valve

ABSTRACT

A valve especially suitable as a relatively cheaply manufactured means for controlling liquids in laboratory equipment, comprises a body having a valve seat with a flow passage passing through the seat, and a spindle located within the body; the spindle comprising a mandrel having an annular groove, a resilient annulus located in the annular groove, and an inert sheath fitted closely over the surface of an end portion of the mandrel and overlying the resilient annulus, the sheath having a closed end shaped to engage the seat for occlusion thereof and having overlying the resilient annulus an annular sealing portion standing proud of the surface of the rest of the sheath, the annular sealing portion being biased against the surrounding body to form a sliding gland seal. The sheath may be preformed or moulded in situ, and protects the mandrel and resilient annulus from attack by fluids flowing through the valve.

This is a continuation of application Ser. No. 617,510 filed Sept. 29,1975, now abandoned, as a continuation of application Ser. No. 482,983filed June 25, 1974, now abandoned.

The invention relates to valves for the control of fluid flow, andespecially to valve spindles which can be easily and cheaplymanufactured for controlling liquids in laboratory equipment such asburette taps.

According to the invention there is provided a valve for the control offluid flow, comprising a body having a valve seat with a flow passagepassing through the seat, and a spindle located within the body; thespindle comprising a mandrel having an annular groove, a resilientannulus located in the annular groove, and an inert sheath fittedclosely over the surface of an end portion of the mandrel and overlyingthe resilient annulus, the sheath having a closed end shaped to engagethe seat for occlusion thereof and having overlying the resilientannulus an annular sealing portion standing proud of the surface of therest of the sheath, the annular sealing portion being biased against thesurrounding body to form a sliding gland seal.

The end portion of the mandrel over which the sheath is closely fitted,preferably has a cylindrical surface so as to provide good support forthe sidewall of the sheath while minimising dead space between the valvebody and the spindle.

In one preferred form of the valve, the resilient annulus of the spindleis an O-ring having an external diameter greater than that of themandrel so that in overlying the O-ring, an annular portion of thesheath is caused to stand proud of the surface of the rest of thesheath. Such a spindle may be manufactured by locating an O-ring in anannular groove around a mandrel, and inserting the mandrel and O-ringinto a sheath whose internal diameter is less than or equal to thediameter of the mandrel. The diameter of the O-ring being greater thanthat of the mandrel, it distends outwards an annular portion of thesheath slid over it so that the annular portion stands proud of the restof the sheath surface. By distending the sheath in this manner, theO-ring is not only able to bias the annular portion of the sheathagainst the valve body to form the gland seal, but also provides a meansfor retaining the sheath on the end portion of the mandrel.

An alternative method for forming the spindle on such a valve is toinjection mould the sheath onto the mandrel and O-ring, for example byinjection moulding a thermoplastic material, the mould being shaped sothat the sheath follows the contour of the mandrel and the O-ringwhereby the annular portion of the sheath overlying the O-ring ismoulded to stand proud of the rest of the surface. However wheninjection moulding a sheath over the mandrel and proud O-ring from oneend of the spindle, there is a tendency when using a wide groove, forthe O-ring to be pushed to the back of the groove and the front of thegroove filled with the moulding composition. This is generally adisadvantage in that it tends to reduce the resilience of the bias ofthe annular sealing portion against the valve body. It is thereforepreferred to match the sizes of the O-ring and its retaining groove sothat the O-ring is a close fit within the groove to seal the grooveagainst ingression of the sheath-forming material during the moulding ofthe sheath around the mandrel and O-ring.

The above build up of moulding composition is aggravated by the O-ringstanding proud of the mandrel surface, and may be reduced by making theresilient annulus lie flush with or below the level of the mandrelsurface. An alternative form of valve which makes use of this fact, andwhich is accordingly particularly suited to having its sheath formed insitu by injection moulding, is one in which the surface of the resilientannulus lies flush with or below the level of the mandrel surface, andthe sheath has an upstanding annular ridge formed thereon, the ridgeoverlying the resilient annulus and providing the proud annular sealingportion.

In all these spindles, whether formed by using a preformed sheath havingan annular portion distended by an O-ring or formed by moulding thesheath in situ, it is preferable to provide means for retaining thesheath on the mandrel. Whether this is strictly necessary depends partlyon the materials used for the sheath, O-ring and mandrel, and partly onthe pressure gradient across the gland seal when the valve is in use.Thus for example, when using materials with very low coefficients offriction for the sheath (e.g. PTFE), or where there is to be a muchlower pressure within the valve than without (e.g. in evacuatedequipment) the provision of specific retaining means becomes moreimportant. Under the reverse conditions, the friction between the sheathand the mandrel may be sufficient to retain the sheath. For conditionsof low pressure differential, such as may usually be found in a burettetap for example, the outward distension of the sheath by a proud O-ring,may be the only means required for retaining the sheath on the mandrel.Where other retaining means are required, these may suitably includemeans to key the sheath to the mandrel. These are most easy to obtain inspindles wherein the sheath is moulded in situ, by providing furtherdepressions (such as another annular groove) in the surface of thespindle and moulding into the depression a key integral with theremainder of the sheath.

In all forms of the present valve, the sheath isolates the mandrel andresilient annulus from any fluid flowing through the valve. Both themandrel and the resilient annulus may therefore be formed from thephysically most appropriate material without any considerations beingnecessary of what fluid is to flow through the valve and whether itmight corrode such materials. Suitable materials include for examplebrass or glass-filled nylon for the mandrel and rubber for the resilientannulus. The material used for the sheath however, needs to be inertwith respect to the fluid since the fluid and sheath will be in contactduring usage of the valve. Materials which may be suitable for formingthe sheath include for example, nylon and polyolefines. However, becauseof this inertness to a wide spectrum of chemical compounds,fluorine-containing polymers are generally preferred. These also havethe advantage of low coefficients of friction which facilitate operationof the valves. Of the fluorine-containing polymers,polytetrafluoroethylene-hexafluoropropylene copolymers (FEP) may be usedto preform a sheath or to injection mould a sheath in situ, andpolytetrafluoroethylene (PTFE) is particularly suitable for preformedsheaths.

The invention is illustrated by specific embodiments of both forms ofthe invention shown by way of example in the drawings, in which

FIG. 1 is a section through a burette tap in which the spindle wasformed using a preformed sheath,

FIG. 2 is a section through a mandrel having alternative features, andwhich is suitable as a replacement spindle for the tap of FIG. 1,

FIG. 3 is a section through a valve spindle having a sheath formed by insitu injection moulding, and

FIG. 4 is a section through a burette tap like that of FIG. 1 butincorporating a spindle (not sectioned) as shown in FIG. 3.

The valve shown in FIG. 1 has a glass body comprising a stem 1 having asmooth surfaced bore and with a valve seat 2 at one end. The other endof the stem is externally threaded 3. Inlet and outlet arms 4 and 5 openinto the bore of the stem, one via the valve seat. The valve spindle hasa mandrel 11 having a unitary end portion having a cylindrical surfacewith an annular groove 12 in the surface. Within the groove is located aresilient annulus in the form of a rubber O-ring 13, whose outerperipheral part stands proud of the cylindrical surface of the mandrel.Enclosing the end of the mandrel is a close fitting PTFE sheath 14having thin sidewalls 15 and a more massive closed end 16 shaped toengage the valve seat 2. At the other end, the mandrel has two lugs 17,and mounted on the end of the mandrel is a cap 18 having slots toreceive the lugs 17. The cap is retained in place by a screw 19.

In assembling the spindle, the O-ring 13 was first slipped over the endof the mandrel until it was located in the groove 12. While holding theclosed end of the sheath by hand, the sidewalls defining the open end ofthe sheath were warmed gently to expand them, and while still warm theywere pushed over the mandrel and O-ring until the mandrel was touchingthe closed end portion of the sheath, or nearly so. The cap 18 was thenplaced on the end of the mandrel and secured in place by the screw 19.On insertion of the spindle into the bore of the stem so that the capthreads engaged those of the body 2, the assembly of the valve wascomplete, and the valve was ready to be incorporated into laboratoryapparatus. The valve was then operable in known manner by rotation ofthe cap, which by its screw-threaded engagement of the valve body movedthe spindle axially towards or away from the valve seat 2.

An annular portion of the sidewalls of the sheath was distended outwardsby the O-ring and pressed against the internal surface of the bore ofthe stem thereby providing the gland seal of the valve. When viewed fromoutside, that is through the glass valve body, the appearance of theseal was that of an annulus of about 1 mm in width wherein the glasslooked as though it was wetted by the PTFE where this was pressedagainst it by the O-ring.

PTFE is the material preferred for a preformed sheath of this kindbecause of its well known chemical inertness and low coefficient offriction. However, we find it also possesses physical properties whichare very well suited to this application. Thus it is sufficientlyresilient to provide adequate closure of the valve seat and to followminor variations in the bore wall when forming the annular gland seal.Our previous experience with such low friction materials when testing asimilar structure for the use as a piston exposed to high vacuum, wasthat during operation the sheath tended to be sucked off the end of themandrel unless positively secured in some manner. However, we now findsurprisingly that with the more limited movements which are usuallyfound in valve operation, the sheath may be retained with quite adequatetenacity for many laboratory applications without the provision offurther retaining means.

We made up several valves of different sizes, all essentially of thesame shape as that illustrated. The valves tested had bore diametersranging from 7.16 mm (0.282 inch) to 25.40 mm (1.00 inch), this rangebeing selected as covering the majority of commonly used laboratoryglassware. All had PTFE sheaths fitted onto glass-filled nylon mandrelshaving annular grooves 0.94 mm ±0.04 mm (0.037±0.0015 inch) deep and2.03 mm (0.080 inch) wide with rubber O-rings having a 1.78 mm (0.070inch) thickness. In respect of these materials and dimensions, we foundthat the internal diameter of the sheath was preferably equal to or lessthan the diameter of the mandrel before assembly, for adequate retentionof the sheath in applications such as burette taps, but for more arduousconditions, it is preferable to use a sheath of smaller internaldiameter. A mandrel having a diameter of 1 to 3% greater than theinternal diameter of the sheath before assembly is more generally to bepreferred. The sidewall of the PTFE sheath where it overlies the O-ring,preferably has a thickness less than 0.63 mm (0.025 inch) thick becausethicker PTFE walls were found to grip the O-ring less firmly. Withsidewall thicknesses below 0.3 mm (0.012 inch), the sheaths becamedifficult to handle and so it is preferred to use sheaths in which thesidewalls are at least 0.3 mm (0.012 inch) thick. Most suitable internaldiameters for standard rubber O-rings of nominal thickness 1.78 mm(0.070 inch) are 80-90% of the mandrel diameter at the base of thegroove.

A valve shaped substantially as illustrated in FIG. 1 was tested to findthe efficiency of the gland seal produced by the annular portion of thePTFE sheath which is pressed against the bore of the valve body. Thevalve had the following dimensions: bore diameter 7.16 mm (0.282 inch),PTFE sheath outside diameter 6.86 mm (0.270 inch), sheath sidewallthickness 0.51 mm (0.020 inch), thickness of the O-ring 1.78 mm (0.070inch), internal diameter of the O-ring 3.68 mm (0.145 inch) and diameterof the mandrel at the base of the groove 3.81 mm (0.150 inch). The testwas carried out by using a well known technique in which the article isconnected to a large evacuated vessel and the time taken for thepressure to rise between two specified pressures is measured. In thepresent test, one arm of the valve was sealed while the other arm wasconnected to a 1 l vessel, the valve being in the open position. Thevessel was provided with means to evacuate it and a Pirani gauge tomeasure the pressure in it. After normal degassing procedures, the glandseal formed in the stem was found to have a leak rate better than 5×10⁻⁷torr dm³ s⁻¹ when the pressure within the vessel rose from 10⁻⁴ to2×10⁻⁴ torr.

The O-ring used in the valve subjected to the above leak rate tests wasa standard size presently commercially available. This thickness of1.78±0.08 mm (0.070±0.003 inch) is generally suitable for mostapplications found in normal laboratory apparatus. Other thicknessescurrently available are 2.62 mm (0.103 inch) and 3.53 mm (0.139 inch),and these may be used where the other dimensions permit. Thus althoughgood seals might be producible with the larger thicknesses even on smallvalves, an excessive depth of groove would weaken the mandrel and beunsuitable for mechanical considerations. In general, therefore,standard 1.78 mm (0.070 inch) thick O-rings are preferred for valves inlaboratory apparatus. The O-ring used in the above tests had a circularcross-section, these being more generally available, but O-rings havingother cross-sectional shapes appear to give good results also.

The valve was designed as a simple valve, suitable for controllingliquid flow through laboratory glassware, with the intention that itshould be reasonably cheap to mass produce commercially. Accordingly adesign requiring only a small quantity of the relatively expensive PTFEhas been provided, and no provision for adjusting the outward distensionhas been made in order to reduce the number of parts to be made andassembled. Hence although the resilience of the O-rings allows a smallamount of tolerance in the diameter of the bore of the stem the size ofthe annular distended portion should be matched to the bore diameter. Inpractice for controlling liquid flow at ambient pressures, e.g. in aburette, the lack of precision on standard tubing sizes does not usuallyprovide any problems, and, as the above results show, where the sizesare suitably matched, it is possible to obtain an extremely good sealeven with high pressure differentials across the seal. However wheretemperature cycling is required, especially with a high pressuredifferential across the gland seal, it is preferred to replace thespindle of the present invention with a more sophisticatedadjustably-distensible spindle, e.g. as described in British PatentSpecification No. 1,157,620.

It is not essential to use PTFE for the sheath, and thermoplasticmaterials in particular are readily formed into very cheap sheaths, buttoo flexible a material may not be sufficiently stiff to be retained inplace solely by the distension due to the O-ring. For example we havemade an experimental valve having dimensions similar to those of thevalve specifically described hereinabove and whose leak ratemeasurements have been described, in which the PTFE sheath was replacedby a polypropylene sheath. The sheath was retained satisfactorily andprovided an adequate gland seal. This valve was stiffer to operate thanthe one using PTFE, but because of the low gearing provided by the screwcap illustrated, it was fully satisfactory in use.

Using a two part mould having a cylindrical chamber with an annularchannel part way along its length, we have also made valve spindles byinjection moulding a sheath over a mandrel and O-ring. The mandrel andO-ring were as shown in FIG. 1 except that the mandrel had a step on theside of the O-ring remote from the nose, the step increasing the radiusby the thickness of the sheath. The mandrel and O-ring were then placedinto the mould with the O-ring located in the plane of the annularchannel of the mould. Polypropylene was injected into the mould from thespindle nose end and formed an even layer over the mandrel and O-ring upto the shoulder. Hence, just as the O-ring stood proud of the mandrel,the even layer of the sheath by following the contours of the mandreland O-ring, likewise had an annular portion standing proud of the rest.Although some flash was visible where the two mould parts met, the proudsealing portion of this spindle also appeared to "wet" the glass evenlyaround the circumference, and the resultant burette tap was found tobehave in a manner very similar to that using the preformedpolypropylene sheath described hereinabove.

FIG. 2 shows in section a mandrel having two independent features notshown in FIG. 1, each of which can be advantageously incorporated. Theseare an air bleed tube 21 and an undercut boss 22 at the cap end of themandrel. The other features which are common to FIG. 1 have been givenlike numerals. The air bleed tube is a tube for connecting the end ofthe mandrel enclosed by the sheath, with the atmosphere, so that duringassembly the trapped air may escape more easily than it does past theO-ring. It is generally most convenient to have an axial tube as shown,but provided it enables air to escape it can take alternative paths. Theundercut boss is provided to receive the cap with a snap action. The capmay be the same as that used in FIG. 1, and the snap fit avoids the needfor the screw 19, which would otherwise require the mandrel to be boredand tapped to receive the screw.

The shoulders 17 are provided to prevent rotation of the mandrel withrespect to the cap. In most cases this is not essential, and normallythe spindle may be made free to slide along the bore without rotationalmovement corresponding to that of the cap when the latter is rotated toprovide the axial movement by its threaded engagement with the valvestem. Where rotation of the mandrel is not essential, the shoulders maybe omitted.

An alternative form of valve to that illustrated which is also found inlaboratory glassware, is one in which the two arms (equivalent to theillustrated arms 4 and 5) are aligned, and the valve stem which containsthe spindle is perpendicular to them. The valve is then closed byinserting the spindle between the two tubes so that they are both closedsimultaneously by the sides of the spindle. However, to be efficient,such valves tend to be more expensive because the seating to receive thespindle on closure requires accurate forming, and is generally moreexpensive to produce than a seat formed at the mouth of a single armcoaxial with the spindle and aligned so as to be closable by the end ofthe spindle, as shown in FIG. 1. The latter valve having the single armclosable is therefore preferred where a cheap valve is required.

The spindle of FIG. 3 comprises a mandrel 31 moulded from glass-fillednylon, and inset into an annular groove around the mandrel is an annulus32 of rubber whose surface is flush with the surface of the mandrel.Adjacent the annulus 32 is a further annular groove 33. Around the endof the mandrel is a sheath 34 which was injection moulded in situ frompolypropylene, the closed end of the sheath being shaped to form amassive nose 35, and at its end remote from the nose, the polypropylenehas flowed during moulding into the groove 33 to form a key 36. Aroundthe spindle and overlying the annulus 32, is a ridge 37. The end of thespindle remote from the sheath is axially drilled and tapped and has ashoulder 38 extending outwards on both sides.

In FIG. 4, the spindle is shown located inside a valve body 41, havingtwo arms 42,43, and a valve seat 44. A cap 45 is secured to the spindleby a screw 46 and fits closely over the two shoulders 38. The cap isthreaded and engages threads 47 on the body, so that when the cap isrotated the spindle nose moves towards or away from the seat 44. Closureof the valve is effected by moving the spindle nose 35 into engagementwith the seat 44. The internal diameter of the valve body was 0.415 inchand the diameter of the spindle ridge was 0.430 inch, so that when thespindle was inserted into the valve body, the ridge was forced inwardsagainst the bias of the rubber annulus 32 which therefore constantlyurged the ridge outwards against the valve body to form the gland seal.The purpose of the gland seal is to prevent any fluid flowing throughthe valve from escaping along the side of the spindle towards the cap,and hence the sheath 34 in extending from the gland-seal-forming ridgeand around the nose of the spindle as a continuous layer, effectivelyisolates the mandrel and its annulus from the fluid whose flow is beingcontrolled.

I claim:
 1. A method for the production of a spindle for a fluid flowcontrol valve comprising a body having a valve seat with a flow passagepassing through the seat and a spindle slideable within the valve bodyto bring one end into engagement with the valve seat for occluding theflow passage during operation of the valve; the method comprisingforminga mandrel with an end portion thereof having a substantially cylindricalsurface with an annular groove therearound, insetting into said groove aresilient annulus with its external surface substantially flush with thecylindrical surface of the mandrel, and injection moulding an inertthermoplastic material around said end portion of the mandrel to form acontinuous sheath therearound, the part of the mould overlying theresilient annulus being shaped to provide around the sheath an integralannular ridge of diameter greater than the internal diameter of thevalve body,whereby on assembly of the valve, the annular ridge is biasedagainst the valve body by the underlying resilient annulus to therebyform a gland seal for preventing loss of fluid from the flow passage.